As exemplified by Japanese laid-open publication No. 11-198433, an organic light emitting device is applied to a print-head for forming an image on a photo conductor of an image forming apparatus such as a printer, as well as to a display. The print-head 11, as shown in FIG. 9, is designed so that plural sets of an organic EL element drive circuit 9 and an organic EL element ELD are disposed in line abreast, with the number of sets being several thousand or more. One of the plural organic EL element drive circuits 9 has a configuration as shown in FIG. 10, for example.
Referring to FIG. 10, a work of the organic EL element drive circuit 9 will be explained here. Switching elements FET1, FET2, and FET3, and a capacitor CN, which are shown in FIG. 10, constitute a current copier circuit 4.
In this configuration, a current source 10 is set to a specific current value. A trigger signal terminal is set to the level ‘H’ to turn ON the switching elements FET2 and FET3. A lighting strobe signal terminal 3 is set to the level ‘L’ to turn OFF a switching element FET6. By sending an electric current from the current source 10 to the capacitor CN through the switching elements FET2 and FET3 for a specific time, the capacitor CN is charged with an electric charge necessary for lighting the organic EL element ELD. After that, the trigger signal terminal 2 and the lighting strobe signal terminal 3 are set to the level ‘L’ for a short time (until the electric charge is charged in the capacitors of all the organic EL element drive circuits 9 of the print-head 11), and the electric charge is retained in the capacitor.
Next, when the trigger signal terminal 2 is set to the level ‘L’ while the lighting strobe signal terminal 3 is set to the level ‘H’, the switching elements FET2 and FET3 are turned OFF while the switching element FET6 is turned ON, whereby the electric charge retained in the capacitor CN is flowed into the organic EL element, and the electric charge thus flowed lights the organic EL element ELD. At this time, the switching element FET1 apparently becomes the same condition as a diode, and the electric charge equivalent to that for the capacitor CN is flowed from the capacitor CN to the organic EL element ELD through the switching elements FET1 and FET6.
In addition, a power supply terminal 1 is connected with a plus power supply for driving the switching elements constituting the drive circuit. The switching element may be any one of A FET and a transistor.
For the printer, each organic EL element is lighted or extinguished by sending signals to each organic EL element of the print-head according to an input image, whereby an electrostatic latent image is formed on the photo conductor.
In case of the production of the organic EL element included in this light emitting device, regardless of the use of the light emitting device, there are occasions when a portion without emitting light, which is generally called a ‘black spot’, appears in the organic EL constituting the organic EL element ELD. In most cases, this is a portion where an organic EL layer becomes thin due to the injection of foreign substances in the production process. Since the portion has a very low electric resistance as compared with the other organic EL layer, the electric charge concentrates on this portion. Accordingly, if a current with a given value is fed hereto, it is hard to obtain the desired luminous intensity. Moreover, owing to the concentration of the electric charge, the black spot slowly destroys the organic EL layer surrounding itself, and this causes a sharp reduction of the luminous intensity in the course of time. In the worst case, it brings about a destruction of the organic EL element ELD.
To solve such problem, there is a suggestion such as organic EL element drive circuit 9 capable of applying a reverse bias to an organic EL element in order to destroy a defective structural part with a low-resistance and remove the black spot, as shown in FIGS. 11 and 12.
The circuit shown in FIG. 11A includes an electric potential switch circuit consisting of three switching elements, FET8, FET9, and FET10, as a reverse bias application circuit.
In this circuit, when the lighting signal terminal 6 is set to the level ‘H’, the switching elements FET7 and FET10 are turned ON while the switching elements FET8 and FET9 are turned OFF through an inverter INV. And at that time, a current IC1 for lighting the organic EL element ELD is flowed to the organic EL element through the switching elements FET7 and FET10, whereby the organic EL element ELD is lighted.
Subsequently, when the lighting signal terminal 6 is set to the level ‘L’, the switching elements FET7 and FET10 are turned OFF while the switching elements FET8 and FET9 are turned ON, as shown in FIG. 11B. And at that time, a cathode of the organic EL element ELD is connected to the plus power supply 1 through the switching element FET8. In addition, an anode of the organic EL element ELD is connected to the ground through the switching element FET9. Hereby, the organic EL element ELD is biased in reverse. If the organic EL element ELD has a failed structural part, a current IC2 suddenly flows through the failed structural part in a reverse direction to the current IC1 that flows from the cathode to the anode of the element.
In this way, the reverse electric field is applied to the organic EL element ELD from the plus power supply 1 through the switching elements FET8 and FET9, whereby the black spot structural part and the peripheral part burn out, and this prevents the failed part from extending any more. As a result, it is possible to ensure the luminous intensity of the organic EL element and to extend the life of the organic EL element.
The circuit shown in FIG. 12 includes a power supply switch circuit for switching from a plus power supply 7 to a minus power supply 8 in order to apply the reverse bias to the organic EL element.
In this circuit, when the lighting signal terminal 6 is set to the level ‘H’, the switching element FET11 is turned ON while the switching element FET12 is turned OFF thorough the inverter INV connected with the lighting signal terminal 6, as shown in FIG. 12A. Through the switching element FET11 thus turned ON, the current IC1 for lighting the organic EL element ELD is applied on the organic EL element ELD from the plus power supply 7.
Subsequently, when the lighting signal terminal 6 is set to the level ‘L’, the switching element FET11 is turned OFF while the switching element FET12 is turned ON though the lighting signal terminal 6, as shown in FIG. 12B. Through the switching element FET 12 thus turned ON, the anode of the organic EL element ELD is connected to the minus power supply 8, with the result that the reverse bias is applied on the organic EL element ELD. Also in this case, likewise, if the organic EL element ELD has a failed structural part, a current IC2 suddenly flows through the failed structural part in the reverse direction to the current IC1 that flows from the cathode to the anode of the element. If the power supply sources are switched, the black spot structural part can also burn out in the same way as the electric potential switch circuit.
However, the above-noted prior art needs a circuit to use the power supply voltage as the reverse bias like the electric potential switch circuit and the power supply switch circuit. In addition, since the reverse bias is applied forcibly on the organic EL element by the power supply, the reverse bias becomes excessive, which might destroy the light emitting element itself.